Atmospheric Chemistry and Physics Discussions
www.atmos-chem-phys-discuss.net
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2007
10.5194/acpd-7-5515-2007
http://www.atmos-chem-phys-discuss.net/7/5515/2007/
http://www.atmos-chem-phys-discuss.net/7/5515/2007/acpd-7-5515-2007.html
http://www.atmos-chem-phys-discuss.net/7/5515/2007/acpd-7-5515-2007.pdf
5515
5552
2007-04-25
Diagnosis of processes controlling water vapour in the tropical tropopause layer by a Lagrangian cirrus model
C. Ren
A. R. MacKenzie
C. Schiller
G. Shur
V. Yushkov
Environmental Science Department, Lancaster University, Lancaster, UK
Forschungszentrum Jülich, Germany
Central Aerological Observatory, Dolgoprudny, Russia
We have developed a Lagrangian air-parcel cirrus model (LACM), to diagnose
the processes controlling water in the tropical tropopause layer (TTL). LACM
applies parameterised microphysics to air parcel trajectories. The
parameterisation includes the homogeneous freezing of aerosol droplets, the
growth/sublimation of ice particles, and sedimentation of ice particles, so
capturing the main dehydration mechanism for air in the TTL. Rehydration is
also considered by resetting the water vapour mixing ratio in an air parcel
to the value at the point in the 4-D analysis/forecast data used to generate
the trajectories, but only when certain conditions, indicative of convection,
are satisfied.
These conditions are imposed to confine what processes contribute to rehydration.
The conditions act to restrict rehydration of the Lagrangian air parcels to
regions where convective transport of water vapour from below is significant,
at least to the extent that the analysis/forecast captures this process. The
inclusion of hydration and dehydration mechanisms in LACM results in total
water fields near tropical convection that have more of the "stripey"
character of satellite observations of high cloud, than do either the ECMWF
analysis or trajectories without microphysics.
<br><br>
The mixing ratios of total water in the TTL, measured by a high-altitude
aircraft over Brazil (during the TROCCINOX campaign), have been reconstructed
by LACM using trajectories generated from ECMWF analysis. Two other
Lagrangian reconstructions are also tested: linear interpolation of ECMWF
analysed specific humidity onto the aircraft flight track, and
instantaneous dehydration to the saturation vapour pressure over ice along
trajectories. The reconstructed total water mixing ratios along aircraft
flight tracks are compared with observations from the FISH total water
hygrometer. Process-oriented analysis shows that modelled cirrus cloud events
are responsible for dehydrating the air parcels coming from lower levels,
resulting in total water mixing ratios as low as 2 μmol/mol. Without
adding water back to some of the trajectories, the LACM and
instantaneous-dehydration reconstructions have a dry bias. The
interpolated-ECMWF reconstruction does not suffer this dry bias, because
convection in the ECMWF model moistens air parcels dramatically, by pumping
moist air upwards. This indicates that the ECMWF model captures the gross
features of the rehydration of air in the TTL by convection. Overall, the
ECMWF models captures well the exponential decrease in total water mixing
ratio with height above 250 hPa, so that all the reconstruction techniques
capture more than 75% of the variance in the measured total water mixing
ratios over the depth of the TTL.
We have therefore developed a simple method for re-setting the total water in LACM using the ECMWF-analysed specific humidity in regions where the model predicts convection.
By picking up the main contributing processes to dehydration and rehydration in the TTL,
LACM reconstructs total water mixing ratios along aircraft flight tracks
at the top of the TTL, close to the cold point, that are always in
substantially better agreement with observations than
instantaneous-dehydration reconstructions, and better than the ECMWF analysis
for regions of high relative humidity and cloud.
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